Transparent resin sheets having a surface unevenness formed by a mold, transparent resin sheets coated with a light-diffusing agent or light-scattering agent on the back surface, and similar resin sheets, have been known as light diffusible resin sheets.
Transparent resin sheets subjected to shading printing, shadow printing, or the like printing, and transparent resin sheets having an unevenness in the interior have been known as artistic design sheets.
Japanese Examined Utility Model Publication (Kokoku) No. 7-36742 discloses a luminous decorative sheet prepared by successively laminating a transparent resin layer, a transparent embossed resin layer and a light reflective layer. However, the light reflective layer is laminated to the portion having an uneven pattern in the transparent embossed resin layer. As a result, the sheet is not suited to a light guide which is used by making light incident on the back surface or side of the sheet.
Japanese Unexamined Patent Publication (Kokai) No. 3-256735 discloses a cut-glass-like sheet which is a composite sheet prepared by forming V-shaped grooves on one side of a polyvinyl chloride sheet, and bonding a polyvinyl chloride sheet having a smooth surface to the V-shaped groove-formed surface and which has a closed air layer in the V-shaped groove portion. However, since such a sheet having a structure with an inner air layer shows a large difference in refractive indexes between the resin layer and the air layer, the light divisibility becomes too large. The sheet is, therefore, not suitable for a light guide.
On the other hand, as an instance of a back surface light source device used for a liquid crystal display device, a sign board, a traffic sign, etc., there is an edge-lit-type back surface light source device in which a linear light source is arranged on an end surface of a plate-like light guide. Such an edge-lit-type back surface light source device is one wherein a plate-like transparent material such as an acrylic resin plate is used as a light guide, light from a light source arranged on one side end thereof is made incident thereon, and light is allowed to exit in a plane form from the surface (light-exiting surface) thereof. The light guide used herein usually has a light-scattering portion and/or light-diffusing portion formed on the surface or on the back surface. Such a surface light source element is required to ensure the uniformity of the luminance of the exiting light regardless of the distance from the light source. The performance is particularly important to a large size surface light source element.
Japanese Unexamined Patent Publication (Kokai) No. 5-127159 discloses a surface light source element having a light guide the surface of which is printed with a light-diffusing material such as titanium white in a dot form, and a prism sheet is placed on the light-exiting surface. uniform luminance can be obtained from such a surface light source element by changing the covering ratio of the dots according to a distance from the light source. However, the dot-like pattern must be shielded with a light diffusible sheet. Consequently, the luminance is lowered, and the structure of the surface light source element becomes complicated.
Japanese Unexamined Patent Publication (Kokai) No. 2-84618 discloses a surface light source element having a light guide with a satin surface at least on one of the surface (light-exiting surface) and the back surface, and a prism sheet placed on the light-exiting surface. Although a very high luminance can be obtained from such a surface light source element, the uniformity of the luminance of the exiting light is not satisfactory.